I would skip it unless you were already planning to eat at one of those restaurants for dinner anyways.
Crowd levels should be low, so as long as you get to the theatre 30-45 minutes before the show, you should get seats. If you want front and center, go more with the 45 end.
The only one of the dinner package restaurants I've been to was the Brown Derby. I really liked my meal there, but I'm not sure how appealing it would be to tweens and teens.